The typical subscriber's telephone line is a private line. A private line is a dedicated line from the telephone switching office to the telephone set or equipment in the subscriber's home or office. Access to the line is private since its use is limited to that equipment of the single subscriber. No other party has access to the line. The ringing signal applied to a private line to ring the telephone equipment is typically a constant-frequency polarity-distinctive signal which is offset by a predetermined positive or negative bias voltage from ground. Almost universally, the standard ringing signal applied to a private line is a negative poled 20 Hertz ringing signal applied to the ring conductor of the telephone line.
Party telephone lines are present in some locations, primarily because the number of subscribers within a given geographic area do not justify the added wiring costs of extending individual private lines to each of the consumers. Rural and less populated areas are examples of locations where party lines usually exist. A party line is a single telephone line to which a multiplicity, for example up to four, subscribers are connected. All of the subscribers have access to the line, but only one can use it at a time. The line is not confidential, in that other subscribers can listen in and participate, but usually out of courtesy, do not.
One major consideration regarding a party line is the ability to ring only a specific subscriber by a ringing signal applied over the single party line to which the multiplicity of different telephone sets of different subscribers are connected. One known technique is to allow all the sets of the subscribers to ring, but a different ringing or sounding pattern identifies the subscriber to which the incoming call is addressed. Another more satisfactory approach is that disclosed in U.S. Pat. No. 4,741,024. This patent describes a selective ringing circuit which is connected between the party line and the telephone set, and which responds only to a selected one of a plurality of polarity-distinctive ringing signals applied to the line, to cause only the telephone set of an addressed subscriber to ring in response to the ringing signal. The sets of the other subscribers do not ring in response to the ringing signals except the one addressed to that particular subscriber. Furthermore, the selective ringing circuit described in U.S. Pat. No. 4,471,024 allows conventional telephone equipment designed to be used with private lines to be employed on party lines.
The ringing signal to which the selective ringing circuit of the U.S. Pat. No. 4,741,024 responds is a conventional constant-frequency, polarity-distinctive ringing signal. The selective ringing circuit of the U.S. Pat. No. 4,741,024 patent selectively operates in response to the polarity of the ringing signal on a specific one of the two conductors of the telephone line. The frequency of the signal establishes only the frequency of the audible sound emitted from the ringer of the telephone set, and this frequency is constant, regardless of the polarity of the ringing signal.
Some telephone companies do not distinguish party line ringing signals by voltage polarity. Instead, frequency-distinctive party line ringing signals are employed. For example, 20 and 30 hertz ringing signals may be supplied over either of the conductors of a party line, in order to individually identify and address up to four party line subscribers. The selective ringing circuit described in U.S. Pat. No. 4,741,024 is not frequency responsive, but responds only to the polarity distinctive ringing signals. The selective ringing circuit of the U.S. Pat. No. 4,741,024 patent cannot, therefore, be selectively utilized to achieve comparable advantages in telephone systems using frequency-distinctive ringing signals, as it can be used in those telephone systems utilizing polarity-distinctive ringing signals of constant frequency.